Question

What is the most widely accepted theory on how photosynthesis evolved from non-photosynthesis based life?

According to NAI's expert at Arizona State University, no widely accepted theory exists so far. A popular one is that it could have evolved from phototaxis, or movement toward light or energy, in hot vent communities (using the black body radiation to find the vent). Chemo-osmotic coupling using respiration (with acceptors other than O2) seems to be older than photosynthesis, and it seems the same is true for autotrophy/CO2 fixation. To get photosynthesis, the organism would have needed only to couple the energy of incoming photons to the formation of the proton gradient which is the energy source for making ATP, the cellular energy currency. All the other cellular machinery was already in place (CO2 fixation, making ATP from proton gradients). It seems possible that the system - light energy triggering the passing of electrons from an energized pigment molecule to an acceptor - was first used as a sensing mechanism, and subsequently incorporated into a single photosystem cyclic electron transport scheme (like in Rhodospirillum). Later, linear electron transport along a membrane, systems to interface with electron donors, and the addition of a second photosystem responsive to a different wavelength of light evolved. While there are reasonable scenarios for the latter steps, the first remain rather murky.
May 1, 2002